From the abstract: Across many studies subjective well-being follows a U-shape in age, declining until people reach middle-age, only to rebound subsequently. Ill-being follows a mirror-imaged hump-shape. But this empirical regularity has been replaced by a monotonic decrease in illbeing by age. The reason for the change is the deterioration in young people’s mental health both absolutely and relative to older people. [...] We replicate the decrease in illbeing by age across 34 countries, including the United States and the United Kingdom, using five ill-being metrics for the period 2020-2024 and confirm the findings.
From the conclusions: The world appears to have changed in the face of three major shocks – the Great Recession and Covid as well perhaps as a huge technological change – the coming of social media. [...] These [first] two shocks appear to have lowered the well-being of those under age forty-five relative to older groups and especially those under twenty-five. [...] The technology shock involved the rapid rise in smartphones and internet access, starting in 2011. The deterioration in the mental health in English speaking countries appears to be correlated with the rise in screen time (Blanchflower and Bryson, 2024c).
A comment: I'm very much wary of the impact of social media on mental well-being; at a personal level, treating the big platforms as write-only has done wonders for my own. But I'm not sure that can explain changes in relative well-being between young and middle-aged people. TikTok might be emotionally hazardous, but surely so are Twitter and Facebook? I lean towards a rational expectations explanation: compared with middle-age people, the young give — rationally have to give — more weight to expectations for entry-level jobs (availability, wages, growth opportunities) older people no longer apply to and to long-term social and ecological sustainability their parent's generation might, I believe erroneously, think is sufficiently assured during their lifetimes. Increased economic inequality, less upwards social mobility, technological and economic pressure against entry-level jobs, and deteriorating political and ecological infrastructures – those are all factors that, while negative for most of the population as a whole, impact especially hard on younger people.
In other words, young people's mental well-being has gotten worse than middle-aged people because the world has gotten worse in ways that are particularly bad for them. It's not a good long-term sign for a civilization when that happens.